Photomasks are used in the semiconductor industry to transfer microscale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer. Generally, a photomask is comprised of a transparent substrate and an opaque material. More specifically, a typical binary photomask is comprised of a quartz substrate and chrome opaque material that includes an integral layer of chrome oxide anti-reflective material (AR). The pattern of the chrome opaque material and chrome oxide AR material on the quartz substrate is a scaled negative of the image desired to be formed on the semiconductor wafer.
To create an image on a semiconductor wafer, a photomask is interposed between the semiconductor wafer which includes a layer of photosensitive material and an energy source commonly referred to as a Stepper. The energy generated by the Stepper passes through the portions of the quartz substrate of the photomask not covered by the chrome opaque material and the chrome oxide AR material and causes a reaction in the photosensitive material on the semiconductor wafer. Accordingly, energy from the Stepper is inhibited from passing through the areas of the photomask in which the chrome opaque material and chrome oxide AR is present. The chrome oxide AR material prevents most of the incident energy from being reflected back into the Stepper. If excess energy is reflected back into the Stepper a degraded image will be created in the photosensitive resist material on the semiconductor wafer surface, thereby resulting in a degradation of performance of the semiconductor device.
A finished photomask used in the production of semiconductor devices is formed from a xe2x80x9cblankxe2x80x9d photomask. As shown in FIG. 1, a blank photomask is comprised of four layers. The first layer 2 is a layer of quartz, commonly referred to as the substrate, is typically approximately one quarter inch thick. Affixed to the quartz substrate 2 is a layer of chrome opaque material 4 which typically is approximately 900 angstroms thick. An integral layer of chrome oxide anti-reflective material (AR) 6 is formed on top of the layer of chrome opaque material 4. The third layer of chrome oxide AR material is typically approximately 100 angstroms thick. A layer of photosensitive resist material 8 resides on top of the chrome oxide AR material 6. The photosensitive resist material 8 is typically a hydrocarbon, the actual composition and thickness of which is well known in the art.
The desired pattern of chrome opaque material to be created on the photomask may be defined by an electronic data file loaded into an exposure system which typically scans an electron beam (E-beam) or laser beam in a raster fashion across the blank photomask. One such example of a raster scan exposure system is described in U.S. Pat. No. 3,900,737 to Collier. As the E-beam or laser beam is scanned across the blank photomask, the exposure system directs the E-beam or laser beam at addressable locations on the photomask as defined by the electronic data file. The portions of the photosensitive resist material that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble. As shown in FIG. 2, after the exposure system has scanned the desired image unto the photosensitive resist material, the soluble photosensitive resist is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material 10 remains adhered to the AR material 6.
As illustrated in FIG. 3, the exposed chrome oxide AR material and the underlying chrome opaque material which is no longer covered by the photosensitive resist material is removed by an xe2x80x9cetchingxe2x80x9d process such that only the portions of chrome AR material 12 and chrome opaque material 14 corresponding to the remaining photosensitive resist material 10 remain on quartz substrate 2. This initial or base etching may be accomplished by either a wet-etching or dry-etching process both of which are well known in the art. In general, wet-etching process uses a liquid acid solution to eat away the exposed AR and chrome. A dry-etching process, also referred to as plasma etching, utilizes electrified gases, typically a mixture of chlorine and oxygen, to remove the exposed chrome oxide AR material and chrome opaque material.
A dry etching process is partially anisotropic or directional in nature, rather than the isotropic wet-etching process typically used in the base etching step of photomask manufacture. As shown in FIG. 4, the dry-etching process is conducted in vacuum chamber 20 in which gases, typically chlorine and oxygen, 22 are injected. Also included in vacuum chamber 20 is anode 24 and cathode 26. The electrical field created between anode 24 and cathode 26 form a reactive gas plasma 30 from the injected chlorine and oxygen gases 22. Positive ions of the reactive gas plasma 30 are accelerated toward photomask 28, which is at the same potential as cathode 26, and which is oriented such that the surface area of quartz substrate 2 is perpendicular to the electrical field. The directional ion bombardment enhances the etch rate of the chrome opaque material and chrome oxide AR material in the vertical direction 32 but not in the horizontal direction (i.e., the etching is partially anisotropic or directional).
The reaction between the reactive gas plasma 30 and the chrome opaque material and chrome oxide AR material is a two step process. First, a reaction between the chlorine gas and exposed chrome oxide AR material and chrome opaque material forms chrome radical species. The oxygen then reacts with the chrome radical species to create a volatile which can xe2x80x9cboil offxe2x80x9d thereby removing the exposed chrome oxide AR material and the exposed chrome opaque material. When dry-etching is used for the initial or base etching of the photomask, the hydrocarbon photosensitive resist material can react with the oxygen in the plasma gases limiting the amount of oxygen that can be injected into chamber 20 and used to form the reactive plasma gas 30. Accordingly, in the prior art the amount of gases injected into the vacuum chamber is typically 75 percent chlorine and 25 percent oxygen by volume.
As shown in FIG. 5, after the etching process is complete the remaining unexposed photosensitive resist material is subsequently removed or stripped, using a method well known in the art, leaving a pattern of exposed chrome oxide AR material 12 and chrome opaque material 14 remaining on the quartz substrate 2 conforming to the image initially defined in the electronic data file loaded into the exposure system.
The dimensions of the chrome opaque material on the finished photomask are then measured to determine whether or not critical dimensions are within specified tolerances. Those skilled in the art will appreciate that the critical dimensions of a finished photomask can be more accurately measured after the photosensitive material has been stripped away since the presence of the photosensitive resist material interferes with the taking of accurate critical dimensions measurements. Critical dimensions may be measured at a number of locations on the finished photomask, summed, and then divided by the number of measurements to obtain a numerical average of the critical dimensions. This obtained average is then compared to a specified target number (i.e., a mean to target comparison) to ensure compliance with the predefined critical dimensions specifications. Additionally, it is desired that there be a small variance among the critical dimensions on the substrate. Accordingly, the measured critical dimensions typically must also conform to a specified uniformity requirement.
As shown in FIG. 5, measured critical dimensions may fall outside of required limits because excess chrome opaque material 16 remains on the substrate (i.e., the photomask is under processed). Additionally, CDs may fall outside required limits because too much chrome material has been removed (i.e., the photomask is over processed). If the critical dimensions of a finished photomask are outside the specified tolerances because the finished photomask has been over processed, the finished photomask is rejected and as it cannot be modified to bring it within specified tolerances. Likewise, if the critical dimensions of a finished photomask are outside of specified tolerances because the photomask is under processed, the photomask cannot be re-etched by the methods known in the art because the photosensitive resist material as been removed. Accordingly, if the critical dimensions of a finished photomask after having its photosensitive resist material stripped are not within the specified tolerances (i.e., the photomask is either over processed or under processed) the photomask is xe2x80x9crejectedxe2x80x9d, resulting in a negative cost and schedule impact.
Accordingly, it is the object of the present invention to provide a method for re-etching photomasks thereby improving the yield of photomasks (i.e., reducing the percentage of rejected photomasks) by allowing the critical dimensions of under processed photomasks to be modified after the layer of photosensitive resist material has been removed.
It is a further object of the present invention to provide a method for re-etching under processed photomasks after the photosensitive resist material has been removed so that all critical dimensions are adjusted uniformly.